Waveguide tubes are commonly used structures for combining or splitting a power in a radio-frequency band such as a microwave band or a millimeter-wave band. For example, the radio-frequency power combining using a waveguide circuit has been performed to implement the high output power capability of radio-frequency power sources or radio-frequency transmitters. A prior art document concerning such a waveguide circuit is, for example, Patent Literature 1 (Japanese Patent Application Publication No. 2005-159767).
Patent Literature 1 discloses a branch structure of a waveguide structure for the radio-frequency power combining or splitting. In the branch structure, the end portion of a first waveguide tube and a second waveguide tube are arranged to be orthogonal and overlapped with each other. In the overlapped area between the end portion of the first waveguide tube and the second waveguide tube, a coupling window formed in a sidewall of the end portion is in communication with a coupling window formed in a sidewall of the second waveguide tube. The branch structure, upon receiving radio-frequency powers through both ends of the second waveguide tube, is capable of combining the received radio-frequency powers to thereby generate a composite power, and outputting the composite power to the first waveguide tube through the coupling windows. Thus, the branch structure can combine two input radio-frequency powers to generate a single output composite power.